The present invention relates to an improved reproduction apparatus and method, but more particularly to treating an electrostatic latent image on a photoreceptor so that an improved copy of an original document is produced.
In conventional xerography, a xerographic plate or photoreceptor comprising a layer of photosensitive insulating material (e.g., selenium) affixed to a conductive backing (e.g., nickel) is used to support electrostatic latent images. In the xerographic process, the photosensitive surface is electrostatically charged, and the charged surface is then exposed to a light pattern of the image being reproduced to thereby discharge the surface in the areas wherein light strikes the surface. The undischarged areas of the surface thus form an electrostatic charge pattern (an electrostatic latent image) conforming to the original pattern. The latent image is then developed by contacting it with a finely divided electrostatically attractable powder referred to as "toner." Toner is held on the image areas by the electrostatic charge on the surface. Where the charge is greater, a greater amount of toner is deposited. Thus, a toner image is produced in conformity with a light image of the copy being reproduced. Generally, the developed image is then transferred to a suitable transfer member (e.g., paper), and the image is affixed thereto to form a permanent record of the original document.
In the practice of xerography, the transfer member is caused to move in synchronized contact with the photosensitive surface during the transfer operation, and an electrical potential opposite from the polarity of the toner is applied to the side of the paper remote from the photosensitive surface to electrostatically attract the toner image from the surface to the paper.
For various reasons, copies of an original occasionally contain undesirable low density image areas which detract from the quality of the copy. For example, low density lines are produced when a copy is made of an original which has had correction tape applied thereto to block out matter which is not to be reproduced. Depending on the thickness of the correction tape, or on how close together adjacent pieces of correction tape are, various low density lines are produced on the copy which correspond to the edges of the correction tape. This occurs because soft shadows are formed along the edges of the correction tape by the illumination assembly used to expose the original, and thus significantly less light is transmitted from the original to the photoreceptor where these shadows are located. Thus, the charged photoreceptor is not discharged to background level in these areas, and consequently these areas of the photoreceptor are developed when the latter passes the developing apparatus. Undesirable low density areas on copies are also produced by a scratched photoreceptor. These scratched areas of the photoreceptor charge to a higher level than the unstratched areas and then discharge to a higher level than the unscratched areas, and consequently these scratched areas of the photoreceptor are developed along with the image which one wishes to reproduce. Thus, these scratched areas of the photoreceptor are reproduced on the copies as undesirable low density image areas. Undesirable low density image areas may also be produced on copies simply because the original is not of good quality and contains such areas.
Also, although present high-speed electrostatic reproduction machines make excellent quality half-toner copies which are faithful reproductions of a half-tone original, they produce very poor copies of coninuous tone photographs. Copies are produced with very high contrast between light and dark grey areas of the photograph. In addition, some applications require precise reproduction of the character width and darkness on the original, and present reproduction machines do not provide for this.